Power lines conventionally carry 60 Hertz alternating current which produces alternately increasing and collapsing magnetic fields, and thereby induces longitudinal 60 Hertz voltages and lower harmonics of these voltages in the conductors enclosed within telephone cables. This interferes with the signals carried by the conductors, and is becoming more and more of a problem with the changes resulting from growing use of electronic and digital switches in telephone systems.
Cable shielding as presently used is practically ineffective in reducing these induced voltages. Other remedial measures, such as neutralizing transformers, longitudinal chokes, and longitudinal drains, are very effective. However, their use on large cables, from 100 to 2100 pairs or larger, becomes very expensive because these devices operate on individual pairs of conductors in a cable, and it is necessary to treat every pair in the cable to produce the most satisfactory results.